1. Field of the Invention
The present invention relates to a high-purity carbon fiber-reinforced carbon composite (C/C composite) and a method for producing the high-purity carbon fiber-reinforced carbon composite.
2. Description of the Related Art
A carbon fiber-reinforced carbon composite (hereinafter also referred to as a C/C composite) has advantages of being light in weight, being strong in strength and being able to easily grow in size. Therefore, it is widely used for a silicon single crystal pulling-up apparatus, a silicon polycrystal manufacturing apparatus, a semiconductor manufacturing apparatus and the like.
In such uses, high-purity C/C composites refined by using a halogen gas is used, in order to suppress contamination of silicon single crystals and the like with impurities such as metals.
A C/C composite is obtained by shaping carbon fibers or graphite fibers to form a formed article by a method such as filament winding, impregnating the formed article with a binder containing a thermosetting resin such as a phenol resin, and curing and burning it. The obtained C/C composite is refined with a halogen gas, and thereafter, used for the above-mentioned usage (see JP-A-10-152391 and JP-A-2002-173392). The thermosetting resin used in impregnation becomes amorphous glassy carbon in the course of burning.
However, according to the related-art production method, sufficient strength can not be obtained by single impregnation, so that impregnation, curing and burning processes are repeated several times, thereby obtaining sufficient strength.
Further, the related-art C/C composite is low in gas permeability. The reason for this can be considered as follows. A matrix of the C/C composite is mainly composed of amorphous glassy carbon, which has a gas impermeable property. Moreover, impregnation is repeated several times, so that continuous pores leading from a surface layer of the composite to an inside thereof become extremely thin, or a part of the pores become a closed pore not leading to an outside thereof. Therefore, it is difficult to remove impurity mainly composed of metal elements included in the inside of the composite by the related-art refining method of heating the composite in the halogen gas.
In order to solve the above problem, it has been studied to mainly use a crystalline carbon-based powder material such as coke or graphite as a third component of the C/C composite so as to decrease the proportion of the glassy carbon in a product (see JP-A-7-48191 and WO2006/003774).
Such a three-component C/C composite includes: a small amount of the glassy carbon, which is originally gas-impermeable, and impurities inside of which are hard to be removed; the high-density crystalline carbon-based material; the low-density carbon fibers; and the low-density glassy carbon for binding these, which is obtained by carbonizing a phenol resin or the like. Accordingly, the three-component C/C composite can have more voids in the inside thereof than the related-art two-component C/C composite including the low-density carbon fibers and the low-density glassy carbon, even when the three-component C/C composite has the same bulk density as the related-art two-component C/C composite. Therefore, it is considered that a refining gas (halogen gas) can penetrate into the inside of the composite, thereby making it easy to remove the impurities mainly including metal elements.
Table 1 shows physical properties of one example of a three-component C/C composite and one example of a two-component C/C composite. Further, FIG. 1 shows a graph of pore distribution of a three-component C/C composite measured by a mercury penetration method using a porosimeter manufactured by Thermo Finnigan, FIG. 2 shows a graph of pore distribution of a two-component C/C composite, FIG. 3 shows a graph of cumulative pore distribution of the three-component C/C composite and the relate-art two-component C/C composite, FIG. 4 shows a graph of X-ray diffraction measurement of the three-component C/C composite, and FIG. 5 shows a graph of X-ray diffraction measurement of the two-component C/C composite.
TABLE 1Three-ComponentTwo-ComponentC/C CompositeC/C CompositeTrue density (g/cm3)2.241.86Bulk Density (g/cm3)1.571.65Porosity (%)24.510.9X-Ray Diffraction Measurement0.40.6(002) Half-Value Width (°)Tensile Strength in Fiber Orientation87121Direction (MPa)
FIG. 1 reveals that in the three-component system, pores having a radius of about 10 μm concentrate. FIG. 2 reveals that in the two-component system, pores are widely distributed between 0.1 and 10 μm. And, FIG. 3 reveals that the three-component system has more pores compared to the two-component system. Further, Table 1 reveals that the three-component system is higher in porosity than the two-component system because of its high true density, although the three-component system and the two-component system have substantially equal level of bulk density.
FIGS. 4 and 5, and Table 1 show that the three-component C/C composite has a higher crystallinity compared to the two-component C/C composite since the peak intensity on the 002 plane of the three-component C/C composite is sharp. In other words, it can be said that the true density of the three-component C/C composite is higher than that of the two-component C/C composite.
Table 1 reveals that even the three-component system has a tensile strength in a fiber orientation direction exceeding 50 MPa. That is, the three-component system has a strength sufficient for use as a member of a semiconductor manufacturing apparatus.
Further, in the three-component C/C composite, the content of a binder such as a phenol resin which produces a large amount of volatile matter in raw materials is small, so that the high-density C/C composite can be obtained by single impregnation without repeating impregnation. Therefore, it is advantageous that the processes can be substantially simplified as compared to the two-component system.